This invention relates generally to the manufacture of generators and, specifically, to improving the joint between a hydraulic header clip or fitting and a liquid cooled armature bar.
The armature windings of large steam turbine generators are generally water-cooled. The armature windings are composed of half coils or armature bars connected at each end through copper or stainless steel fittings and water-cooled connection rings to form continuous hydraulic circuits. The hydraulic winding circuits are connected to inlet and outlet water manifolds with plastic hoses to provide electrical isolation. The manifolds are connected to the stator water cooling system which cools, filters and deionizes the water and pumps the water back to the armature winding. The armature bars are each composed of rectangular copper (or other suitable metal or alloy) strands arranged in rectangular bundles or packages. The copper strands can be all hollow strands or a mixture of solid and hollow strands. The hollow strands provide a duct for conducting cooling water. The strands are joined to each other and to a waterbox end fitting or header clip at each end of an armature bar. The header clip functions to deliver water to and collect water from the hollow strands. The header clip is connected through copper or stainless steel fittings to a second armature bar to form a complete armature coil element of the winding. The joints between the strands and the header clip must retain hydraulic and electrical integrity for the 30-year plus expected lifetime of the winding. The filler metal or braze alloy that joins the strands and the header clip is constantly exposed to a deionized, oxygenated water environment and corrosion of the filler metal and adjoining strand surfaces can occur under certain conditions. The corrosion process can initiate if the joint surface contains surface crevices, pinholes, or porosity, and the critical water chemistry conditions that can support corrosion develop within these indications. The corrosion process can continue through the braze joint as long as critical crevice and water chemistry conditions exist. Underlying porosity within the braze joint can accelerate the corrosion rate. Eventually, the path of corrosion can result in water leakage through the joint compromising the hydraulic integrity of the header clip-to-strand joint.
In a previous approach to solving the problem, all of the strands were cut to the same length and the filler metal or braze alloy was pre-placed flush to the ends of the strands. A braze alloy anti-wetting agent was used on the ends of the strands to prevent plugging of the hollow strands and an inert purge gas was used during the brazing cycle. (See U.S. Pat. No. 5,796,189). Use of the anti-wetting agent, although effective for preventing hollow strand plugging, resulted in a high probability of causing potential corrosion-initiation sites on the surface of the braze joint, discontinuities in surface wetting, and crevices between braze alloy fillets and hollow strands. It was also observed that the anti-wetting agent could be improperly applied and, through capillary action, flow between strands, resulting in strand faying surface contamination and a poor effective braze joint length. The inert purge gas limited oxidation during the brazing cycle but provided no de-oxidizing benefits.
This invention provides for a brazed connection between a liquid-cooled armature bar strand package and a hydraulic header clip having significantly reduced potential corrosion-initiation sites on the joint surface and longer effective braze joint lengths. The invention also eliminates the need for an anti-wetting agent and employs a de-oxidizing purge gas which improves braze alloy wetting and flow.
In the exemplary embodiment, the resulting clip-to-strand braze joint contains extended hollow strands which ensures that the hollow strand openings will not become plugged during brazing. This arrangement also eliminates the need to use a potentially contaminating filler metal anti-wetting agent on the ends of the hollow strands, and provides smooth, continuous fillets of braze alloy around all strands and between the strand package and the header clip. It also reduces voids on the braze joint surface and the invention also results in lower porosity within the joint due to the de-oxidizing benefits of the purge gas.
In practicing the invention, a special tool may be employed to shorten the solid strands at each end of the strand package, but other suitable techniques may also be employed. In addition, a filler metal or braze alloy is used that is designed to fill both small and large gaps and wetted surfaces, and is pre-placed flush with the ends of the shortened solid strands. Finally, a de-oxidizing forming or purge gas is used to purge the armature bar during the brazing cycle. A cover gas micro-environment is formed within the header clip cavity to maximize the deoxidation of faying surfaces prior to the braze alloy melting and flowing. The temperature of the strand package and header clip are increased using a pre-programed time-temperature profile until the braze alloy melts, flows, and fills the braze joint in a single-step, automated brazing cycle. The de-oxidizing forming gas purge is continued until the joint cools to a non-oxidizing level.
In its broader aspects, therefore, the invention relates to a brazed joint between an armature bar strand package and an end fitting comprising a plurality of solid strands and a plurality of hollow strands arranged in an array and forming the strand package, the plurality of hollow strands having free ends that extend axially beyond corresponding free ends of the solid strands; a cavity in the end fitting accessed by an opening, the free ends of the plurality of hollow strands and the corresponding free ends of the solid strands extending through the opening and received in the cavity; and a braze alloy joining the free ends of the plurality of solid strands and the corresponding free ends of the plurality of hollow strands to each other and to interior surfaces of the end fitting, wherein the braze alloy, prior to melting, does not extend axially beyond the free ends of the solid strands.
In another aspect, the invention relates to a brazed joint between an armature bar and an end fitting comprising an array of solid and hollow strands arranged in an array, the hollow strands having free ends that extend axially beyond corresponding free hollow ends of the solid strands, wherein the solid strands and the hollow strands are present in a ratio of from 1 to 1 to 6 to 1; a cavity in the end fitting, accessed by an opening, the array received in the opening; and a braze alloy joining the solid and hollow strands to each other and to internal surfaces of the end fitting, the braze alloy not extending substantially beyond the free ends of the solid strands.
In still another aspect, the invention relates to a method of forming a brazed joint between an armature bar and an end fitting comprising: providing an armature bar having a strand package comprising a plurality of hollow strands and a plurality of solid strands such that free ends of the hollow strands extend axially beyond free ends of the solid strands; pre-placing a braze alloy on the free ends of the hollow strands and the solid strands such that the braze alloy does not extend axially beyond the free ends of the solid strands; inserting the strand package and braze alloy through an opening and into a cavity in the end fitting; purging the cavity with a de-oxidizing purge gas; and heating the armature bar strand package and the end fitting to a temperature sufficient to melt and flow the braze alloy.
The invention will now be described in connection with the drawings identified below.